1. Field of the Invention
The present invention relates to an optical waveguide device, an optical modulator, a mounting structure for an optical waveguide device and a supporting member for an optical waveguide substrate.
2. Related Art Statement
Recent development of multimedia systems demands a broadband communication network system. An optical transmission system operating on a frequency higher than 10 Gb/s has been already commercialized. Such a system capable of operating on a higher frequency is expected. For example, a lithium niobate optical modulator or a semiconductor filed absorption modulator (Electro-absorption modulator: EA modulator) is used as a device for modulating an electrical signal (microwave signal) with a frequency of not lower than 10 Gb/s.
A lithium niobate optical modulator is composed of a Mach-Zehnder type optical waveguide and a travelling wave type electrode. In such a modulator, for realizing a high frequency modulation of a microwave signal with a frequency not lower than 10 Gb/s, it is important and indispensable to satisfy a velocity matching condition of the microwave signal and optical wave. Historically, the velocity matching has been satisfied by providing a thicker electrode or by forming a layer made of a low dielectric constant between a substrate and electrode, so as to reduce the effective refractive index of the microwave signal.
Alternatively, a thinner optical waveguide substrate may be provided with a thickness of as small as 10 μm for satisfying the velocity matching condition at a higher frequency. Thickness of 10 μm, however, is the same level as the size (half value width: 1/e2) of the optical waveguide. Therefore, the light spot propagating in the optical waveguide may be flattened so as to increase the connecting loss with an outer fiber connected with the waveguide. In addition to this, the surface roughness of the waveguide substrate may affect the light propagating in the waveguide so as to increase the propagating loss of the light. The assignee disclosed the following invention in Japanese Patent laid-open publication 133, 159A/1998 for providing a solution of the above problems. In the invention, a thicker portion including an optical waveguide and a thinner portion adjacent to the thicker portion are provided in an optical waveguide substrate used for a travelling wave type optical modulator. The thickness of the thinner portion is adjusted to a small value of, for example, 10 μm. It is thereby possible to prevent the deterioration of the optical insertion loss, and to realize a high frequency optical modulation without the necessity of a buffer layer of silicon oxide. Further, a product (Vπ•L) of a driving voltage Vπ and length of an electrode L may be advantageously reduced.